The present invention relates generally to the field of antennas and in particular to a new and useful directional antenna that is steerable by configuring a switched plasma, semiconductor or optical crystal screen surrounding a central transmitting antenna.
Traditionally, antennas have been defined as metallic devices for radiating or receiving radio waves. Therefore, the paradigm for antenna design has traditionally been focused on antenna geometry, physical dimensions, material selection, electrical coupling configurations, multi-array design, and/or electromagnetic waveform characteristics such as transmission wavelength, transmission efficiency, transmission waveform reflection, etc. As such, technology has advanced to provide many unique antenna designs for applications ranging from general broadcast of RF signals to weapon systems of a highly complex nature.
Included among these antennas are omnidirectional antennas, which radiate electromagnetic frequencies uncontrolled in multiple directions at once, such as for use broadcasting communications signals. Usually, in the absence of any additional attennas or signal attenuators, an omnidirectional radiation lobe resembles a donut centered about the antenna. Antenna arrays are known for producing a directed transmission lobe to provide more secure transmissions than omnidirectional antennas can. Known antenna arrays require many powered antennas all sized appropriately to interfere on particular frequencies with the main transmitting antenna radiation lobe, and thereby permit transmission only in the preferred direction. Antenna arrays normally have a significant footprint, which increases greatly as the angular width of the transmission lobe is reduced.
Generally, an antenna is a conducting wire which is sized to emit radiation at one or more selected frequencies. To maximize effective radiation of such energy, the antenna is adjusted in length to correspond to a resonating multiplier of the wavelength of frequency to be transmitted. Accordingly, typical antenna configurations will be represented by quarter, half, and full wavelengths of the desired frequency.
Plasma antennas are a newer type of antenna which produce the same general effect as a metal conducting wire. Plasma antennas generally comprise a chamber in which a gas is ionized to form plasma. The plasma radiates at a frequency dictated by characteristics of the chamber and excitation energy, among other elements. U.S. Pat. No. 6,369,763 and applicant's co-pending application Ser. No. 10/067,715 filed Feb. 5, 2002 disclose different configurations and applications for plasma antennas.
Efficient transfer of RF energy is achieved when the maximum amount of signal strength sent to the antenna is expended into the propagated wave, and not wasted in antenna reflection. This efficient transfer occurs when the antenna is an appreciable fraction of transmitted frequency wavelength. The antenna will then resonate with RF radiation at some multiple of the length of the antenna. Due to this, metal antennas are somewhat limited in breadth as to the frequency bands that they may radiate or receive.
Recently, wireless communications have become more and more important, as wireless telephones and wireless computer communication are desired by more people for new devices. Current wireless communications are limited to particular ranges of the electromagnetic frequency spectrum. High-speed communications are limited by the selected frequency spectrum and number of users which must be accommodated. For example, 3G networks can presently provide a maximum data transfer rate of up to 2 Mbps, shared among network users.
Also, because most non-line-of-sight wireless communications are now done using omnidirectional antennas, transmissions between wireless communicators may be easily intercepted by an unintended recipient having the correct equipment. Transmissions require data encryption to provide some security, which detracts from computing speed and can increase the amount of data transmitted.
In the case of wireless home networking, for example, it is simple for an unauthorized user to connect via a compatible wireless device due to the omnidirectional nature of the antennas used to transmit and receive the network communications between devices. The unauthorized user can simply situate themselves within the effective distance of the wireless network transceiver, and they can use the omnidirectional transmission lobe to gain access to the wireless network. This inability to limit access by the shape of the area within the wireless network inherent in known wireless networks is one reason for slow acceptance of wireless networks in offices and other work environments where communications security is needed.
Further, because omnidirectional antennas broadcast indiscriminately, an unauthorized user can find an available wireless network to piggy-back on, or worse, break into, using basic signal detection equipment. Antennas can be provided in arrays to limit the radial direction in which an active antenna broadcasts. Arrays rely upon the reflective and absorptive properties of antennas to produce transmission lobes in specific radial directions. Increasingly more antennas are required to produce increasingly narrower lobes and no or smaller side lobes. Larger arrays with more antennas necessarily require more space to work effectively, and therefore have a larger footprint than a single omnidirectional antenna or a small array. Thus, conventional antenna arrays are not practical for home and office wireless communications applications due to their large size requirements for effectively directing the radiation lobes of the broadcasting antenna.
As a result, directional antenna arrays are normally only used in military applications. But, even military applications are limited by the size requirements for direction antenna arrays. While it is relatively simply to install an array on an aircraft carrier, it is essentially impossible to install an effective array on a Humvee or fighter jet, for example. And, changing the transmission lobe direction with an array requires switching antennas in the array between powered and unpowered states. Metal antennas experience a delay during switching, so that changing the transmission lobe direction in an array is not instantaneous.
Therefore, there is clearly a both a civilian and military need for a directional antenna which occupies a relatively small space, can be mobile, and is rapidly configurable to produce a transmission lobe in any direction upon command.
Further, expansion of wireless networking capabilities is needed, as wireless communications become more and more ingrained in daily life.